After an oil/gas well is drilled and completed (so that production may proceed) it may become necessary to access the oil/gas well to perform various “workover” operations. Such workover operations may include a variety of process operations including, but not limited to, replacing various components, stimulating the production from the oil/gas well by chemical treatments, etc. In the case of subsea oil/gas wells, such workover operations are performed through a workover riser that extends from a workover vessel or ship on the surface of the water to the well equipment positioned at the bottom of the sea. In particular, such a workover riser may extend from a surface vessel to a Christmas tree positioned above the wellhead of the subsea well. A riser may also be used in other situations as well, such as when installing a Christmas tree on or above a subsea wellhead.
Typically, in subsea applications, such a workover riser may extend beneath the surface of the water for a very long distance, e.g., 1.5 miles or more, depending upon the depth of the well and the depth of the water. Traditionally, such risers are comprised of multiple tubular components or pipes that are threadingly coupled to one another using pin/box connections. In one embodiment, such a workover riser may be comprised of sections or “stands” of tubular pipes, wherein each stand is comprised of multiple tubular pipe segments that are coupled to one another using a coupling. Multiple such stands of tubulars are sequentially inserted into the water to create the riser. More specifically, when inserting a stand of tubulars for increasing the overall length of the workover riser, the stands of tubulars are sequentially connected to one another as the workover riser is increased in length as it is extended toward the well head at the sea floor. Conversely, in the case where a workover riser is removed from an oil/gas well, each stand of such tubulars is unscrewed from the overall riser string and positioned on the deck of the workover vessel. When inserting or removing a stand of tubulars, the portion of the riser that remains below the vessel is supported by the vessel.
Within the oil gas industry, powered pipe tongs are typically used for threadably engaging and disengaging tubular goods, such as drill pipes, and pipe sections for workover risers, etc. Such tongs typically have hardened metal gripping teeth that bite into and penetrate a surface of the engaged component. In operation, a first tong engages the first of two tubular components to be joined together, while a second tong engages the second tubular that is to be joined to the first tubular. The tongs are then power driven to so as to provide relative rotation between the first and second tongs so as to threadingly couple/decouple the two tubulars to or from one another, respectively.
More specifically, a typical workover vessel includes a platform and power tools such as one or more elevators and a spider that are used to engage, assemble, and lower the stand of tubulars into the water. The elevator is suspended above a floor of the vessel by a draw works that can raise or lower the elevator in relation to the floor of the vessel. The spider is mounted in the floor. The elevator and spider both have so-called “slips” that is capable of engaging and releasing a tubular. The elevator and the spider are designed to work in tandem. Generally, the spider is actuated such that it engages and holds the uppermost stand of the riser so as to support the entire weight of the riser positioned below the vessel while another stand of pipes is added to the workover riser positioned below the vessel. In general, the elevator engages a new stand of tubulars (upper stand) and aligns it over the stand (the lower stand) of the riser that is being held in position by the spider. Thereafter, the tongs, e.g., a power tong and a spinner, are then moved into position so as to physically engage the upper and lower stands of tubulars. At least one of the tongs is then energized to cause the upper and lower stands to rotate relative to one another so as to couple the upper stand and the lower stand together. Once the upper and lower stands of tubulars are coupled to one another, the elevator is then actuated to raise the riser, and the spider is then disengaged from the lower stand. The elevator is then used to lower the riser through the floor until the elevator and spider are at a predetermined distance from each other. The spider then re-engages the uppermost stand of the workover riser and the elevator is then disengaged from the stand of the riser that is now being held by the spider. This process is repeated until such time as the desired overall length of the riser is assembled. As indicated above, this sequence can be reversed to disassemble the riser.
Importantly, the tongs and slips have inserts with teeth that are forced against the wall of the pipe. It is well known in the industry that such tongs and slips mar or penetrate, i.e., create notches or gouges, in the surface of the component that they engage. The presence of such notches, scratches or gouges in the component may set up undesirable stress risers in the pipe. It is also well known that steel fails under repeated loading and unloading, or under reversal of stress, at stresses smaller than the ultimate strength of the steel under static loads. The magnitude of the stress required to produce failure decreases as the number of cycles of stress increase. This phenomenon of the decreased resistance of steel to repeated stresses is called “fatigue” that leads to fatigue cracking.
More recently, oil and gas producers have been drilling deeper wells in deeper water in an effort to maintain or increase their reserves of oil and gas. Although what constitutes an “ultra deep-water” well is a matter of opinion, based upon current technology, ultra deep water-wells are commonly thought to be wells that are drilled in at least 6000 feet of water. Many of such wells drilled in deeper water may also be subjected to “High Temperature High Pressure” (HPHT) conditions, i.e., the operating formation pressures and temperatures within the well. Just like the wellhead components, workover risers for use on such HPHT wells must also be rated for the HPHT service conditions. Yet another variable that must be considered when designing a subsea riser is the nature and characteristics of the hydrocarbons produced from the well. For example, some wells produce hydrocarbons that contain hydrogen sulfide (H2S). Such wells are sometimes referred to as “sour service” wells. Hydrogen sulfide is known to cause stress corrosion cracking in high-strength materials such as high-strength low-alloy carbon steel. In wells that involve production of corrosive materials, such as H2S, alloys such as chromium and/or molybdenum may be added to the materials used for the riser in such applications in an effort to avoid or limit stress corrosion cracking. Operators of “sour service” wells require that riser materials be “NACE qualified” by passing a testing regime specified by NACE MR0175, wherein “NACE” refers to the corrosion prevention organization formerly known as the National Association of Corrosion Engineers, now operating under the name NACE International, Houston, Tex.
All of the aforementioned issues must be addressed when designing risers that are intended for use in connection with a deep-water, HPHT and sour service well. First, for very long risers (required in deep-water applications), the use of low strength materials (yield strength of 85 ksi or less) for the riser components is not acceptable due to the fact that the riser becomes very heavy due to the relatively large thickness of the low strength material that is required to support all imposed loads on the riser. For example, a riser made of such low-strength materials may not be able to support the weight of the riser itself and/or withstand the stresses imposed on such long risers, including being subjected to internal formation pressures during at least some workover operations. Accordingly, risers for deep-water HPHT applications that do not involve sour service wells, may be made of so-called “high-strength” materials, materials having a yield strength of at least 90 ksi so as to reduce the thickness of the various components of the risers, e.g., the pipes, and thereby reduce the overall weight of the riser. For deep-water HPHT wells that are also subjected to sour service conditions, a balancing of various factors is required when designing such risers, as will be discussed more fully below.
FIG. 1 depicts an example of an illustrative stand of tubulars 20 of a workover riser, wherein the riser 20 is manufactured using so-called “high strength” materials, i.e., materials with a yield strength of 90 ksi or greater, sometimes referred to as low-alloy steels. In the depicted example, the stand of tubulars 20 is comprised of two sections of high-strength pipe 22A, 22B, an upper high-strength coupling 24A and a lower, high-strength coupling 24B. FIG. 1 also includes enlarged views of portions of the riser 20. The overall length 29 of the stand of tubulars 20 may vary depending upon the particular application, e.g., about 45 feet. In the depicted example, the overall stand of pipes 20 has an upper box connection 26 and a lower pin connection 28. The nominal diameter of the pipe sections 22A, 22B may vary depending upon the particular application, e.g., 7⅞ inches.
As shown in the enlarged views, upper high-strength coupling 24A also comprises two box connections, the upper one of which serves as the box connection 26 for the overall stand 20, while the lower box connection is coupled to the pin connection of the pipe 22A. Similarly, the lower high-strength coupling 24B also comprises two box connections, the upper one of which is coupled to the pin connection of the pipe 22A, while the lower box connection is coupled to the upper pin connection of the pipe 22B. The high-strength couplings 24A, 24B are couplings that are made to precise specifications and manufactured using known rolling and extrusion manufacturing techniques followed by machining of the threads for the box/pin connections.
Making connections between such high-strength stands of pipe 20 using power tongs can be problematic. In general, power tongs should only come into contact with the couplings 24A, 24B so as to avoid in gouging penetration of the surface of the high-strength pipes 22A, 22B. When joining two stands 20 together, one of the tongs will engage the coupling (24A, 24B) of the first stand, but the other tong must engage the pipe on the other stand. As a result, the surface of the high-strength pipes 22A, 22B becomes scarred, gouged or damaged due to undesired contact or engagement with the teeth of the power tongs. The net result is that the life of the high-strength pipes 22A, 22B may be greatly reduced. Additionally, the coupling 24A does not have any significant shoulder that is useful for engagement by an elevator or a spider. Note that the coupling 24B may be attached to the pipes 22A, 22B in the factory using special equipment, i.e., using protective layers positioned between the tong dies and outside diameter of the pipe Typically, the lifting and makeup of such stands 20 is accomplished by use of devices that have special “non-marking” slips and tongs which do not damage the pipes 22A, 22B. These additional special slips and tongs can cause additional costs and delays as it related to the overall project of frequent installation of a riser for a subsea well.
FIG. 2 is an example of an illustrative stand of tubulars 10 of a workover riser, wherein the riser is manufactured using so-called “high strength” materials, i.e., materials with a yield strength of 90 ksi or greater, such as low-alloy carbon steel. In depicted example, the stand of tubulars 10 is comprised of two sections of high-strength pipe 12A, 12B, an intermediate high-strength coupling 14, an upper, machined, low-strength forging 16 and a lower, machined low-strength forging 18, wherein the forgings 16, 18 are made of a material having a yield strength of 85 ksi or less. FIG. 2 also includes enlarged views of the high-strength coupling 14 and the low-strength forgings 16 and 18. The overall length 19 of the stand of tubulars 10 and diameter of the pipes 12A, 12B may be about the same as those set forth for the riser described in FIG. 1.
As shown in the enlarged view of the upper forging 16, the upper forging 16 is comprised of a forged body 16A, an upper pipe connection 16B, a lower pipe connection 16C, a riser support shoulder 16D, an elevator support shoulder 16E and a tong-engagement area 16F positioned above the elevator support shoulder 16D. The overall axial length of the upper forging 16 may vary depending upon the particular application, e.g., five feet. In the depicted example, the upper and lower pipe connections 16B, 16C, are both box connections. As depicted, the lower pipe connection 16C is coupled to the pin connection on the pipe section 12A. To manufacture the upper forging 16, an initial forging is obtained and various machining operations are performed to define at least the riser support shoulder 16D and the elevator support shoulder 16E in the outer portion of the forged body 16A and to define the axial bore that extends through the body 16A of the upper forging 16 as well as the pipe connections 16B, 16C. The intermediate coupling 14 also comprises two box connections that engage the pin connections on the pipe sections 12A, 12B. The intermediate coupling 14 is typically made to precise specifications and manufactured along with the pipes 12A, 12B using known rolling and extrusion manufacturing techniques followed by machining of the threads for the box/pin connections.
As shown in the enlarged view of the lower forging 18, the lower forging 18 is comprised of a forged body 18A, an upper pipe connection 18B, a lower pipe connection 18C, a support shoulder 18D and a tong-engagement area 18E. The overall axial length of the lower forging 18 may vary depending upon the particular application, e.g., 3-5 feet. In the depicted example, the upper pipe connection 18B is a box connection that is adapted to engage the pin connection on the pipe section 12B. The lower pipe connection 18C is a pin connection that is adapted to engage the box connection 16B on another stand of pipe 10. To manufacture the lower forging 18, an initial forging is obtained and various machining operations are performed to define at least the shoulder 18D in the outer portion of the forged body 18A and to define the axial bore that extends through the body 18A of the lower forging 18 as well as the pipe connections 18B, 18C.
During operations, the support shoulder 16D of the upper forging 16 is engaged by the spider to maintain the entire weight of the riser below the vessel at the surface of the sea. Thereafter, an elevator (not shown) engages the elevator support shoulder 16E on another stand of pipe 10, lowers the pin connection 18C into engagement with the box connection 16B of the pipe section that is engaged by the spider. Thereafter, a lower power tong (or similar torque-generating device) (not shown) is positioned around and engages the surface 16F of the pipe stand 10 that is engaged by the spider, while an upper power tong (or similar torque-generating device) (not shown) is positioned around and engages the surface 18E of the pipe stand 10 that was just positioned above the pipe stand 10 engaged by the spider using the elevator. Thereafter, the power tongs are actuated so as to tighten the connection between the two stands of pipe 10. The elevator is coupled to the now combined stand of pipe 10, the spider is retracted, and the elevator lowers the assembled pipe stands into the water below the vessel.
As mentioned above, it is well known that steel fails under repeated loading and unloading, or under reversal of stress, at stresses smaller than the ultimate strength of the steel under static loads. The magnitude of the stress required to produce failure decreases as the number of cycles of stress increase. This phenomenon of the decreased resistance of steel to repeated stresses is called “fatigue”. The danger of such fatigue cracks appearing is greater if the stress within a material is increased or concentrated due to the presence of a stress concentrator, such as, for example, a local defect such as a notch or significant scratch that penetrates the outer surface of the material, such as defect that is produced when the teeth of power tongs or slips engage a pipe. Once formed, the crack tends to spreads due to the stress concentrations at its ends. This spreading of the crack progresses under the action of the alternating stresses until the cross-section becomes so reduced in area that the remaining portion fractures suddenly under the load.
The present application is directed to a unique coupling for a high strength riser with mechanically attached support members that may eliminate or at least minimize some of the problems noted above.